1. Technical Field
The present invention relates to a method for producing a light-emitting device having an improved light extraction efficiency.
2. Background Art
In general, light emitted from an active layer of a light-emitting device cannot be efficiently extracted since being reflected at an interface due to a difference in the refractive indices of the light-emitting layer, a transparent crystal substrate and the outside air.
In view of the above, there has been developed a light-emitting device designed to improve a light extraction efficiency by suppressing the absorption caused by the multiple reflection by forming the outer surface of the light-emitting device to have an unevenness structure. As a method for forming an unevenness structure, there have been adopted a method for forming an uppermost layer into an unevenness surface during the growth and a method for chemically or physically forming an uppermost layer into an unevenness surface after the growth (see Japanese Unexamined Patent Publication No. H06-291368).
There has been also proposed a method for forming an unevenness in a transparent resin layer coating a light-emitting surface of a light-emitting device. The light-emitting device is joined with a transparent substrate via the coating transparent resin layer, and the resin layer is irradiated with a laser beam through the transparent substrate, whereby the resin layer is formed with an unevenness structure by the laser ablation (Japanese Unexamined Patent Publication No. 2002-368289).
There is also known a method for pressing a mold against a resist formed on a surface of a substrate to transfer an unevenness structure to the resist and transferring the unevenness structure to the substrate by dry etching of a low selection ratio using this resist as a mask (Japanese Unexamined Patent Publication No. 2000-232095).
There is further known a method for applying a SOG film to a surface of a substrate, pressing a mold against the SOG film to transfer an unevenness structure, and transferring the unevenness structure to the substrate by RIE (reactive ion etching) using this SOG film as a resist mask (Japanese Unexamined Patent Publication No. 2003-100609).
However, according to the method for forming the outer surface into the unevenness structure during the growth disclosed in Japanese Unexamined Patent Publication No. H06-291368, an improvement in the light extraction efficient is as low as about 10% since a degree of surface roughness is low. On the other hand, according to the method for chemically or physically forming the outer surface into the unevenness structure after the growth disclosed in the same publication, the shape reproducibility of the unevenness structure is poor since the outer surface is merely made rough by etching or grinding. Further, since the unevenness structure is formed by the laser ablation according to the method disclosed in Japanese Unexamined Patent Publication No. 2003-368289, there is a problem of a poor shape reproducibility of the minute unevenness structure. As a processing method having a good shape reproducibility, an unevenness structure may be formed by dry etching after a pattern is formed by optical lithography. However, it is still difficult to form an unevenness structure having a pyramidal shape, a semispherical shape or any arbitrary shape or minute unevenness structure having a size equal to or below a resolution restricted by wavelength.
In Japanese Unexamined Patent Publication No. 2000-232095, an ion milling method according to which the resist and the semiconductor layer are etched substantially at the same speed is adopted as the dry etching of a low selection ratio. Since a surface to be etched has a strong angle dependency according to the ion milling method, this method can be applied to form a rectangular unevenness structure or unevenness structure having a pitch in the range of from several nm to several tens of nm where the angle does not count, but it is difficult to form a minute unevenness structure having a pitch in the range of from several hundreds of nm to several μm, where the angle is not negligible such as those having a pyramidal shape.
Further, a polymer resist made of, e.g., polymethyl-methacrylate (PMMA) is used as the resist. Since such a resist has a low resistance to dry etching if a dry etching method with a chlorine gas is adopted, it is difficult to process the resist to have minute pyramidal or semispherical unevenness structures or process the resist to have about the same selection ratio.
In Japanese Unexamined Patent Publication No. 2002-100609, the depth range of the unevenness structure of about 100 nm is disclosed in an embodiment. In order to transfer an unevenness structure having pitches or depth in the range of about several μm, a pressure which is at least tenfold of the pressure disclosed in the embodiment is actually necessary, wherefore it is difficult to transfer an unevenness structure of several hundreds of nm to several μm to a light-emitting layer or a transparent crystal substrate (sapphire substrate).